1. Field of the Invention
The invention relates to techniques for making fluorine-doped silica glass for use in optical fiber fabrication.
2. Discussion of the Related Art
Optical fiber is produced from a glass preform. The preform is generally arranged vertically in a draw tower such that a portion of the preform is lowered into a furnace region. The portion of the preform placed into the furnace region begins to soften, and the lower end of the preform forms what is known as the neck-down region, where glass flows from the original cross-sectional area of the preform to the desired cross-sectional area of the fiber. From the lower tip of this neck-down region, the optical fiber is drawn.
Optical transmission fiber typically contains a high-purity silica glass core optionally doped with a refractive index-raising element such as germanium, an inner cladding of high-purity silica glass optionally doped with a refractive index-lowering element such as fluorine, and an outer cladding of undoped silica glass. In some manufacturing processes, the preforms for making such fiber are fabricated by forming an overcladding tube for the outer cladding, and separately forming a rod containing the core material and inner cladding material. The core/inner cladding are fabricated by any of a variety of vapor deposition methods known to those skilled in the art, including vapor axial deposition (VAD), outside vapor deposition (OVD), and modified chemical vapor deposition (MCVD). MCVD is discussed in U.S. Pat. Nos. 4,217,027; 4,262,035; and 4,909,816. MCVD involves passing a high-purity gas, e.g., a mixture of gases containing silicon and germanium, through the interior of a silica tube (known as the substrate tube) while heating the outside of the tube with a traversing oxy-hydrogen torch. In the heated area of the tube, a gas phase reaction occurs that deposits particles on the tube wall. This deposit, which forms ahead of the torch, is sintered as the torch passes over it. The process is repeated in successive passes until the requisite quantity of silica and/or germanium-doped silica is deposited. Once deposition is complete, the body is heated to collapse the substrate tube and obtain a consolidated core rod in which the substrate tube constitutes the outer portion of the inner cladding material. To obtain a finished preform, the overcladding tube is typically placed over the core rod, and the components are heated and collapsed into a solid, consolidated preform, as discussed in U.S. Pat. No. 4,775,401.
Because the outer cladding of a fiber is distant from transmitted light, the overcladding glass does not have to meet the optical performance specifications to which the core and the inner cladding must conform. For this reason, some efforts to both ease and speed manufacture of fiber preforms have focused on methods of making overcladding tubes. One area of such efforts is the use of a sol-gel casting process.
Co-assigned U.S. Pat. No. 5,240,488 (the '488 patent), the disclosure of which is hereby incorporated by reference, discloses a sol-gel casting process capable of producing crack-free overcladding preform tubes of a kilogram or larger. In the process of the '488 patent, a colloidal silicon dispersion, e.g., fumed silica, is obtained. To maintain adequate stability of the dispersion and prevent agglomeration, the pH is raised to a value of about 11 to about 14 by use of a base, and the dispersion is then aged. Subsequent to aging, as discussed in Col. 15, lines 39-65 of the '488 patent, a gelling agent such as methyl formate is added to the dispersion to lower the pH. Typically, once the gelling agent is added, but before gelation occurs, the mixture is pumped into a tubular mold containing a central mandrel, and the gel is aged in the mold for 1 to 24 hours. The mandrel is removed, and the gelled body is then extracted from the mold, typically by launching the body from the mold in water to prevent breakage. The body is then dried, heat treated to remove volatile organic materials, water, and metal and oxide particles, and then sintered to form the finished overcladding tube.
In some applications, such as manufacture of ultra-low loss optical fiber, it is desired to modify the index of refraction of the overcladding tube, typically by doping with fluorine (to lower index) or germanium (to raise index). In particular, it is possible for new fiber designs to require one or more overcladding tubes that exhibit an index of refraction lower than pure silica. Various efforts have been made to find a useful technique for doping glass with fluorine, in order to achieve this index-reduction.
Specifically, previous work has explored both liquid and vapor phase doping of silica glasses. In vapor phase doping, porous silica bodies formed by various processes, e.g., sol gel, Vertical Axial Deposition (VAD), or Outside Vapor Phase Oxidation (OVPO) processes, are exposed to gaseous fluorine or fluorine-containing gases such as silicon tetrafluoride at elevated temperatures. Vapor phase doping, however, is typically slow due to the need for the fluorine-containing gas to diffuse into the pores of the silica body. Vapor phase techniques also tend to be relatively inefficient and potentially expensive because only a small fraction of the fluorine is incorporated in the final glass body. The incorporation of fluorine by vapor phase is governed by the thermodynamic equilibrium of the following reaction: EQU SiF.sub.4 (g)+3SiO.sub.2 (S){character pullout}4SiO.sub.1.5 F(s)
This reaction is typically carried out at temperatures ranging from 900 to 1100.degree. C. and requires an atmosphere of SiF.sub.4 to prevent loss of fluorine from the glass due to the reverse reaction. In particular, heating a fluorine-doped porous body in the absence of such a fluorine-containing atmosphere generally results in unacceptable depletion of the F concentration.
Liquid phase (or solution) doping offers the potential advantage of controlled and uniform levels of doping throughout a glass body. Liquid phase doping using conventional alkoxide sol gel techniques have been reported. (See S. Shibata, "Sol-gel derived silica preforms for optical fibers," Journal of Non-Crystalline Solids, Vol. 178, 272, 1994; Shibata et al., "Fabrication of Fluorine-Doped Silica Glasses by the Sol-Gel Method," Journal of Non-Crystalline Solids, Vol. 100, 269, 1988) This work typically uses fluorinated silicon alkoxides (such as Si(OC.sub.2 H.sub.5).sub.3 F) although reagents such as HF and H.sub.2 SiF.sub.6 have also been used with conventional silicon alkoxides--though with less success (see M. Shilmei et al., "Synthesis of Fluorine Doped Silica and its Defluorination Behavior at High Temperature," High Tech Ceramics, P. Vincenzini, ed., Elsevier Science Publishers, 1987.). Shinmei et al., supra, also report hydrolysis of SiF.sub.4 in water to make F-doped silica particles that are subsequently dehydrated and polymerized to make gels.
However, such liquid phase doping techniques encounter some of the same problems as vapor phase doping Specifically, as mentioned above, in a typical sol gel process for making overcladding tubes or optical fiber preforms, the bodies undergo steps of drying, heating to burnout organic species, heating in the presence species that remove bound water and purify the silica, and finally heating to sinter to glass. As noted above, however, heating of fluorine-doped silica bodies in the absence of a fluorine-containing gas environment causes depletion of the fluorine by the reverse of the above reaction. In fact, Shinmei et al., supra, report that fluorine-doped sol gel powders made by two different solution doping techniques easily lost fluorine when heated in air. In particular, fluorine loss began at temperatures as low as 450.degree. C. and was nearly complete when the sample reached 1000.degree. C. Kitagawa et al., "Fabrication of Single-Mode Optical Fiber Preforms by Sol-Gel Method," Electronics and Communications in Japan, Part 2, Vol. 73, No. 6, 22, 1990, report using a solution doping technique involving forming a sol-gel tube by an alkoxide process, followed by drying and a quick sintering at 1350.degree. C. These authors actually rely on the relatively significant out-diffusion of fluorine at elevated temperatures to form a fluorine-depleted region on the inside surface of the glass cylinder. In this manner, after the tube is collapsed to a rod, the resulting preform contains a silica core with down-doped outer region.
These results suggest that although solution doping with F-containing species is able to provide usable levels of F-doping, subsequent heat treatments to burnout organic species, dehydroxylate, purify and sinter the body will cause loss of F, resulting in a non-uniform and poorly controlled profile, unless steps are taken to maintain a F-containing atmosphere during such heat treatments. Maintaining such an atmosphere is potentially expensive due to the cost of F-containing gases and difficult due to the corrosive nature of these gases.
Note also co-assigned U.S. Pat. Nos. 4,707,174 and 4,840,653 (the '174 and '653 patents, respectively), which discuss both liquid phase and vapor phase fluorine-doping techniques for purposes of avoiding bubble formation during reheating of previously-sintered glass. In one embodiment, the liquid phase doping techniques of these patents involve formation of a silica sol, drying of the sol, and then milling or blending of this pre-dried sol with an aqueous solution of a fluorine-containing material. This embodiment is thus an indirect doping process, in which a silica sol is first formed, and then later doped with fluorine by way of a mechanical mixing process. In another embodiment, a conventional alkoxide sol-gel process involving tetraethyl orthosilicate (TEOS) is performed in the presence of ammonium fluoride, such that the resultant silica contains fluorine. The '174 and 653 patents do not disclose a fluorine-doping technique with a colloidal sol-gel process of the type disclosed in U.S. Pat. No. 5,240,488, i.e., a sol-gel process that involves stabilization of silica particles with TMAH, followed by addition of a gelling agent. Thus, the applicability of the disclosed techniques to such a process is not clear.
Thus, alternative techniques for fluorine-doping in silica sol-gel bodies, advantageously where such techniques do not require a fluorine atmosphere during heat treatments, would be desirable, particularly for sol-gel bodies of the type formed from the process of the '488 patent.